The present invention generally relates to reading of images, and more particularly to an image reading apparatus having a movable optical system for scanning an image.
Generally, there are two distinct types of apparatuses used for image reading, one having a fixed optical system and the other having a movable optical system. In the former type apparatus called a sheet feeding type, the reading or scanning of the image is performed by feeding a sheet or document on which the image to be read is formed, while holding the optical system stationary. In the latter type apparatus called a stationary sheet type, on the other hand, the reading is performed by moving the optical system while holding the sheet stationary.
As the former type apparatus lacks the movable optical system, it can be produced at a low cost, and thus this type apparatus is used commonly as the scanner of a facsimile apparatus and the like. It should be noted that the automatic sheet feeding system used in the former type apparatus can be constructed substantially simpler and cheaper than the movable optical system used in the latter type apparatus.
On the other hand, the latter type apparatus is advantageous, though having a complex construction, in that skewing of the sheet during the sheet feeding does not occur and a high quality image is guaranteed. Further, images such as those formed on the pages of books or magazines, which cannot be read by the sheet feeding type apparatus, can be read easily.
Further, there is another type of image reading apparatus called a dual mode image reading apparatus, wherein the movable optical system is combined with a sheet feeding system so that the reading of the image is made selectively in either a first mode or a sheet feeding mode wherein each of the sheets which carries an image thereon is separated one by one from a stack of sheets and fed one after another, passing through an image reading location while maintaining the movable optical system stationary, and a second mode or a stationary sheet mode wherein the optical system is moved so as to scan over the images on the sheet which is held stationary.
FIG. 1 shows an example of such a conventional dual mode image reading apparatus. Referring to FIG. 1, in the stationary sheet mode, a document or a sheet 2 placed on a contact glass 1 is illuminated by a line- or bar-shaped light source 3 extending parallel to the direction of the horizontal scanning line, and the light reflected back from the image on the sheet 2 is detected by a line image sensor 8 provided at a predetermined position of the apparatus, after a number of reflections by mirrors 4, 5 and 6.
The light source 3 and the mirror 4 are mounted on a slider 9 which is movable parallel to the contact glass 1 in the direction perpendicular to the horizontal scanning line. On the other hand, the mirrors 5 and 6 are mounted on another slider 10 which is also movable in the same direction as the slider 9, though with a speed which is one-half of the speed of the slider 10. Further, a press pad 11 is used for pressing the sheet on the contact glass 1.
In the case of the dual mode image reading apparatus, this press pad 11 is further provided thereon with a predetermined region for placement of a sheet or a stack of sheets to be fed to the image reading location in the sheet feeding mode. In correspondence to this region, a guide part 12 shown in FIG. 1 is provided for guiding the sheet during its feeding.
In the sheet feeding mode, each of the sheets placed on the guide part 12 is separated one by one from the stack by a pair of feed rollers 13, and is fed one after another to a predetermined image reading location A for reading the images by a pair of feed rollers 14. Further, another pair of rollers 15 are provided for discharging the sheet passed through the image reading location A. The sheet thus discharged is collected in a sheet tray 16. Along the path of the sheet, other guide members 17 and 18 are provided.
Further, a home position sensor 19 is provided for detecting that the movable sliders 9 and 10 are located at respective reference positions. These reference positions of the sliders 9 and 10 are usually chosen so as to coincide with the image reading location A, and the sliders 9 and 10 are held stationary at this position as long as the apparatus is operated in the sheet feeding mode.
In the image forming apparatus using the bar-shaped light source 3, there is a general problem, irrespective of whether the image forming apparatus is the sheet feeding type, the stationary sheet type or the dual mode type, in that the optical radiation illuminating the image is not uniform but varies along the elongating direction of the bar-shaped light source 3. Further, the sensitivity of each photosensitive device forming the line image sensor 8 is often scattered device by device. Furthermore, the optical radiation from the bar-shaped light source 3 may be modified significantly, even in an ideal case where the radiation is uniform in the elongating direction of the light source, due to the well known effect of a decrease of light intensity when light is passed through a lens with an offset from the optical axis.
Thus, even when a totally white blank image is read, photosensitive devices forming the line image sensor 8 produce output image signals with various output levels as illustrated in FIG. 2A, in spite of the fact that the output levels of the photosensitive devices should be uniform as shown in FIG. 2B. Such a variation of the output level of the line image sensor 8 is known as "shading".
In order to eliminate the problem of shading, the conventional image reading apparatus generally uses a construction shown in FIGS. 3A and 3B.
Referring to FIGS. 3A and 3B, a part 18a of the guide member 18 facing the contact glass 1 in correspondence to the reading location A is coated white as illustrated by PW, and this white part PW is used as a reference image for defining the white level of the image to be read. Further, an image processing system shown in FIG. 4 is used for processing the output image of the line sensor 8 in combination with the construction shown in FIGS. 3A and 3B.
Referring to FIG. 4, analog output image signals represented as "AV" produced by the photosensitive devices of the line sensor 8 are supplied to an amplifier 20 for amplification, and supplied further to an analog-to-digital converter 21, a bottom hold circuit 22, and a peak hold circuit 23 after amplification. The bottom hold circuit 22 detects the lowest level of the analog output image signals AV for each line of the image and produces an output voltage -Vr indicative thereof. The output voltage -Vr is supplied to the analog-to-digital converter 21 as a first reference voltage, to be described later. Similarly, the peak hold circuit 23 detects the highest level of the analog output image signals AV for each line and produces an output voltage indicative thereof. This output voltage is used to adjust the output of a digital-to-analog converter 25 to be described later, and the output of the digital-to-analog converter 25 is supplied to the analog-to-digital converter 21 as a second reference voltage +Vr.
In the analog-to-digital converter 21, each of the incoming analog image signals AV is converted to a corresponding digital image data DV having a predetermined number of bits. The analog-to-digital converter 21 is supplied thereby with the first reference voltage -Vr and the second reference voltage +Vr, and each of the input image signals AV is normalized with respect to the voltage +Vr and the voltage -Vr prior to the analog-to-digital conversion. In other words, the digital image data DV represents a percentage of the output image signals AV with respect to the maximum value of the signal AV set at +Vr and the minimum value set at -Vr. The output digital image data DV is then supplied on the one hand to a line buffer 24 and on the other hand to a circuit of the following stage. The line buffer 24 stores the output digital image data DV supplied thereto under the control of a controller 101 and supplies the data DV to the digital-to-analog converter 25, also under the control of the controller 101. The digital-to-analog converter 25 converts the digital data DV read out from the line buffer 24 to analog signals and the magnitude of the analog signals thus obtained is further adjusted in proportion with the output of the peak hold circuit 23 to form the reference voltage +Vr.
Hereinafter, the shading correction according to the prior art system of FIG. 4 will be described for the case where the image reading apparatus is operated in the sheet stationary mode.
Referring to FIGS. 1, 3A and 4, the controller 101 moves the sliders 9 and 10 when starting the reading of a sheet, until the home position sensor 19 detects that the sliders 9 and 10 are located at respective reference positions. In this state, the line image sensor 8 reads the white reference image PW formed on the guide plate 18.
During this reading of the white reference image PW, the digital-to-analog converter 25 is controlled by the controller 101 such that the reference voltage +Vr is set to a maximum value that the digital-to-analog converter 25 can produce irrespective of digital data supplied thereto, and reading of the white reference image PW is performed under this state. The proportional adjustment of the reference voltage +Vr by the output of the peak hold circuit 23 is also disabled by the controller 101 during this procedure. On the other hand, the lowest level of the analog image signals AV is detected by the bottom hold circuit 22 and is applied to the analog-to-digital converter 21 as the reference voltage -Vr.
In response to the reading of the white reference image PW as such, the analog image signals AV from the line image sensor 8 are converted to the corresponding digital image signals DV and subsequently stored in the line buffer 24 which is set ready for storing data by the controller 101 at the beginning of reading of the white reference image PW. It should be noted that, in this state, the line buffer 24 stores the white reference level in the form of digital data, and the digital data thus stored in the line buffer 24 reflects the variation of the characteristics of the photosensitive devices as well as the variation of intensity of the light incident to each of the photosensitive devices after the detection of the white reference image PW is made.
Next, the controller 101 moves the sliders 9 and 10 such that the image reading location, which was previously located at the position A, is now located in correspondence to a head part of the sheet 2, and the first line of the document is read by the line image sensor 8. During the reading, the analog-to-digital converter 21 is provided with the reference voltage +Vr from the digital-to-analog converter 25 and further with the output -Vr from the bottom hold circuit 22, and the input analog image signal AV is converted to the digital image signal DV on the basis of these reference voltages. It should be noted that the reference voltage +Vr provided by the digital-to-analog converter 25 is adjusted in accordance with the output of the peak hold circuit 23, and any time-dependent variation of the intensity of illumination by the light source 3, which might have occured since the last reading of the reference white image PW, is compensated.
The obtained digital image signal DV is normalized with respect to the first reference voltage +Vr defining the maximum of the image signal AV and the second reference voltage -Vr defining the minimum, as already described. As the line buffer 24 stores the white reference level, the output image data DV, produced by the analog-to-digital converter 21 using the reference voltage +Vr, is compensated with respect to the variation of the white level. In other words, the effect of the shading is eliminated from the digital image data DV, as shown in FIG. 2B. It should be noted that the controller 101 prohibits the content of the line buffer 24 from being updated once the reading of the image on the sheet is started, until the reading of that sheet is completed and reading of the next sheet is started.
The same procedure is applicable also to the case where the reading of the image is performed in the sheet feeding type image reading apparatus. In this case, the line buffer 24 is set ready for reading by the controller 101 at the beginning of the reading procedure, and the white image PW on the guide plate 18 is read prior to the feeding of the sheet. The digital image data DV thus obtained by the analog-to-digital converter 21 is stored subsequently in the line buffer 24.
Next, the feeding of the sheet is started by driving the feed rollers 14 under the control of the controller 101 (see FIG. 7A), and the image on each of the sheets is read one after another by the line image sensor 8 while the sliders 9 and 10 are held stationary at the respective reference positions. The output analog image signal AV is supplied to the digital-to-analog converter 21 in a manner similar to the foregoing case of the sheet stationary mode and the conversion to the digital image data DV is performed using the white reference level stored in the line buffer 24.
In the foregoing approach for eliminating the shading, there still remains a problem that no correction is applied with respect to the dark current of the line image sensor 8, which may vary in each photosensitive device. Such a variation of the dark current causes a variation of the bottom level or black level, as shown by the hatching in FIG. 5A. Such a variation of the black level is caused also by a flare of light.
Thus, when the foregoing correction is applied only to the white level, as shown in FIG. 5B, the effective output of the line image sensor 8 representing the actual light intensity detected by the line image sensor 8 is varied even when the image read by the sensor 8 is entirely black. Such a variation causes a deterioration of the quality of the image read from the sheet.
In the prior art image reading apparatus, particularly of the sheet feeding type, there exists another problem regarding detection of the size of a sheet.
Referring to FIG. 6 showing a mechanism used in the prior art apparatus for detection of the size of the sheet, there is provided the guide part 12 for guiding the feeding of the sheet comprising a fixed first guide plate 12a and a second guide plate 12b provided movable with respect to the first guide plate 12a in the direction perpendicular to the direction of sheet feeding. In FIG. 6, it should be noted that the direction of feeding of the sheet is perpendicular to the plane of the drawing. Further, the guide plate 12b carries a light shield plate 12c such that the plate 12c is located below the guide plate 12a. Furthermore, there are provided photosensors S1, S2 and S3 below the guide plate 12a such that the light shield plate 12c interrupts incidence of light to the photosensors S1, S2 and S3.
In operation, the movable guide plate 12b is adjusted such that the sheet placed on the guide part 12 is held laterally between the fixed guide plate 12a and the movable guide plate 12b, and in response thereto, the light shield plate 12c selectively interrupts the incidence of light to the photosensors S1, S2 and S3. Thus, the detection of the size of the sheet placed on the guide part 12 is made in response to the output of the photosensors S1, S2 and S3.
In such a conventional mechanism, however, there is a problem in that the resolution or fineness of detection of the size of the sheet is unsatisfactory. More specifically, there arises a problem that the size of the sheet cannot be detected properly when the sheet is not of a standardized sheet size. Further, when sheets having various sizes are mixed, the detection of the size of the sheet is made only for the largest sheet and there arises an inconvenience in that the reading of an image on a sheet having a smaller size is made in a manner similar to the case regarding the reading of an image on the largest size sheet.